Molecular mechanism of autophagy and apoptosis in endometriosis: Current understanding and future research directions

Abstract Background Endometriosis is a common gynecological condition, with symptoms including pain and infertility. Regurgitated endometrial cells into the peritoneal cavity encounter hypoxia and nutrient starvation. Endometriotic cells have evolved various adaptive mechanisms to survive in this inevitable condition. These adaptations include escape from apoptosis. Autophagy, a self‐degradation system, controls apoptosis during stress conditions. However, to date, the mechanisms regulating the interplay between autophagy and apoptosis are still poorly understood. In this review, we summarize the current understanding of the molecular characteristics of autophagy in endometriosis and discuss future therapeutic challenges. Methods A search of PubMed and Google Scholar databases were used to identify relevant studies for this narrative literature review. Results Autophagy may be dynamically regulated through various intrinsic (e.g., PI3K/AKT/mTOR signal transduction network) and extrinsic (e.g., hypoxia and iron‐mediated oxidative stress) pathways, contributing to the development and progression of endometriosis. Upregulation of mTOR expression suppresses apoptosis via inhibiting the autophagy pathway, whereas hypoxia or excess iron often inhibits apoptosis via promoting autophagy. Conclusion Endometriotic cells may have acquired antiapoptotic mechanisms through unique intrinsic and extrinsic autophagy pathways to survive in changing environments.

The most commonly accepted theory is the retrograde menstrual reflux (i.e., Sampson's hypothesis). 2The normal endometrium retrogrades into the peritoneal cavity and is exposed to severe hypoxic stress. 3,4How regurgitated endometrial tissues survive, implant, and grow as endometriotic lesions under harsh conditions is not well understood.Endometriosis is characterized by enhanced proliferation and diminished apoptosis of endometrial and stromal cells. 4Apoptosis has been shown to be regulated by autophagy through several mediators. 5The difference between autophagy and apoptosis can be summarized in two words: "selfeating" and "self-killing," and autophagy is an efficient regulator of apoptosis. 6Autophagy in eukaryotic cells promotes the degradation of subcellular elements to maintain cellular homeostasis via the autophagosome (double-membrane vesicles)-lysosome system. 5,7Intracellular materials degraded by autophagy contain the cargo composed of cytoplasmic components including mitochondria, lipids, oxidized proteins, macromolecules, abnormal protein aggregates, and damaged or aged organelles. 5,8This process induces an alternative source of bioenergetic metabolites and recycles nutrients for survival and cellular protection in response to environmental stress. 3,9For example, it is well accepted that autophagy has been linked to a range of physiological and pathological processes, including reproduction, embryo implantation, 10 myopathy, metabolic disorder, neurodegenerative disease, 11,12 cardiovascular disease, and cancer. 7,9,13tophagy is classified into different categories: macroautophagy, selective autophagy, chaperone-mediated autophagy, and microautophagy. 14Macroautophagy consists of several steps: initiation, induction, phagophore elongation, autophagosome formation and maturation, autolysosome formation, and proteolytic degradation of the contents. 14,15Selective autophagy is the elimination of specific cellular components such as mitochondria (i.e., mitophagy) and requires recognition of injured mitochondria and subsequent activation of autophagy. 14Mitochondria of eukaryotic cells stem from a bacterium 1.5 billion years ago, regulate oxidative phosphorylation, and maintain cellular functions. 16Until now, we have been elucidating the molecular and cellular mechanisms underlying endometriosis pathogenesis, focusing on energy metabolism, mitochondrial dynamics, and cellular redox homeostasis. 17Mitochondria supply adenosine triphosphate (ATP) from aerobic respiration and orchestrate cell proliferation and development, but they also form reactive oxygen species (ROS) as by-products in the electron transport chain. 16,17Increased ROS production causes impairment of mitochondria, so mitophagy has evolved to eliminate the dysfunctional mitochondria. 16,18Thus, mitochondria have evolved mechanisms for quality control and cellular homeostasis, 19 but they also serve as a hub for the apoptosis signaling pathways. 20This is because mitophagy and apoptosis are often induced in response to common stimuli (e.g., mitochondrial dysfunction, oxidative stress, and calcium ion concentration 11 ). 20,21Although autophagy/mitophagy and apoptosis are closely related to each other and share some common signals, they are distinct processes. 21The signal transmission between autophagy/mitophagy and apoptosis in endometriosis is complex and still not fully understood. 22Importantly, autophagy and mitophagy exert opposite functions, protective and lethal, so they can inhibit or promote apoptosis. 21,23Several intrinsic (e.g., genetic predisposition, certain signal transduction pathways, female hormonal stimuli, ATP content, and p53 status 24 ) and extrinsic factors (e.g., hypoxia, oxidative stress, iron concentration, reduced nutrient supply, metabolic stress, and endoplasmic reticulum stress [25][26][27] ) have been reported to be involved in the regulation of autophagy/mitophagy in endometriotic tissues. 5,28Endometriosis cells are thought to control apoptosis by activating or suppressing autophagy/mitophagy in response to both stimuli.In this review, we summarize our current understanding of how autophagy/mitophagy and apoptosis modulate endometriosis development and progression and discuss future directions for research.

| Search strategy and selection criteria
We conducted a narrative review of the literature that focuses on autophagic, mitophagic, and apoptotic function in endometriosis.Electronic databases including PubMed and Google Scholar were searched for literature published up to the October 31, 2023, combining the following keywords: "Apoptosis," "Autophagy," "Endometriosis," "Mitochondria," and "Mitophagy."The search strategy using the keyword string and combination of Boolean operators is shown in Table 1.

TA B L E 1
The search strategy.Papers reporting patients' data and in vitro and animal studies conducted to investigate the potential effect and underlying molecular mechanism were also included.

| THE REG UL ATORY MECHANIS MS OF AUTOPHAGY
Essentially, autophagy exerts its protective effects by adapting to stress conditions and creates a favorable environment for tissue repair and survival. 3,9For example, under conditions of nutrient starvation or hypoxia, autophagy acts as a prosurvival mechanism.

| THE REG UL ATORY MECHANIS MS OF MITOPHAGY
Increasing attention is now being paid to the deregulation of mitochondrial dynamics in endometriosis. 17,40,41The maintenance of mitochondrial biogenesis and homeostasis is achieved by mitochondrial quantity and quality control through continual fusion and fission, i.e., mitochondrial dynamics. 40,41Mitochondrial fusion is driven by the mitochondrial outer membrane dynamin like GTPase fusion proteins, mitofusins 1 and 2 (MFN1 and MFN2), and the mitochondrial inner membrane dynamin like GTPase fusion protein, optic atrophy 1 (OPA1). 42Enhanced mitochondrial fusion facilitates mitochondrial genomic repair and promotes oxidative phosphorylation and generation of ATP, allowing cells to survive even under stressful environments such as starvation. 41On the other hand, eukaryotic cells have evolved mechanisms to maintain cell survival by eliminating dysfunctional mitochondria themselves due to enhanced production of ROS, decreased rates of oxidative phosphorylation, depletion of cell ATP pool, and increased numbers of mitochondrial DNA mutations. 41Mitochondrial fission is crucial for mitochondrial quality control mechanism that eliminates damaged mitochondria via mitophagy. 40,41Mitochondrial fission is primarily mediated by the large GTPase dynamin-related protein 1 (DRP1). 43gulation of the homeostatic balance between mitochondrial fusion and fission is critical for determining mitophagy-mediated cell survival and death.Impaired mitochondrial dynamics or dysfunctional mitophagy is known to be associated with many diseases, including neurodegenerative disorders, 44 metabolic disorders, 45 cancer, 46 and endometriosis. 17
These proteins serve as important mediators that mediate coordination between autophagy/mitophagy and apoptosis. 63Although there are several reports on the role of autophagy-related markers Beclin-1 and LC3 in endometriosis, [65][66][67][68][69][70][71] only a few studies have exploited in vitro and animal models to study the function of the Bnip3 molecule. 15,49Evidence in endometriosis is still scarce regarding the pathways ④, ⑥', and ⑧ (Figure 2).

| MECHANIS M OF INTER AC TION B E T WEEN AUTOPHAGY/MITOPHAGY AND AP OP TOS IS IN ENDOME TRI OS IS
In this section, we summarize how autophagy and mitophagy control apoptosis and the underlying molecular mechanisms, focusing on the intrinsic (Subsections 5.2 and 5.3) and extrinsic (Subsections 5.1, 5.4, 5.5, 5.6, and 5.7) pathways in endometriosis.
Studies have shown that autophagy is associated with the regulation of menstruation and the pathogenesis of endometriosis. 7The autophagy level in the secretory phase was significantly higher than that in the proliferative phase in the stromal cells of the normal endometrium. 72,734][75] Also, the autophagy levels quantitatively differed among distinct endometriotic lesions (ovaries, fallopian tubes, peritoneal, gastrointestinal, and skin). 75Normal endometrial cells with reduced autophagic function may reflux into the peritoneal cavity and proliferate as ectopic endometriotic cells by restricting apoptosis. 4,5,7,74In contrast, other researchers have reported upregulation of autophagy levels in the ectopic endometrium of patients with ovarian endometriosis. 76,77Upregulation of Beclin1 and LC3II expression and downregulation of p62 expression were detected in tissue samples and endometriotic cells from patients with endometriosis. 76,77Therefore, previous studies have yielded mixed or inconsistent results regarding the levels of autophagy in endometriosis.To explore reasons for inconsistent reports, we then focus on how various types of stressors (e.g., signal transduction, oxidative stress, hypoxia, or energy starvation) regulate autophagy-mediated apoptosis in endometriotic cells.
Furthermore, the interlinked connections between autophagy and mitophagy have been reported in endometriosis.A decrease in autophagy activity restores the oxidative imbalance by increasing the expression of nuclear factor (erythroid-derived) 2-like (NRF2) and the antioxidant proteins NAD(P)H quinone dehydrogenase 1 (NQO1) and heme oxygenase 1 (HO1). 50Inhibiting autophagy also restores mitophagy homeostasis and significantly increases the levels of Parkin in a rat model of endometriosis. 50Therefore, well-coordinated quality control mechanisms are essential for mitochondrial homeostasis and adaptation to stress, such as impaired autophagy and apoptosis.

| Hypoxia
When endometrial cells are shed into the peritoneal cavity during menstruation, they face hypoxia.Autophagy is initiated under such stressful conditions and are particularly common in cancer lesions. 87,88In patients with endometriosis, autophagic vacuoles and autophagosomes accumulate extensively within ectopic endometrial cells under hypoxic conditions. 3Hypoxia leads to stabilization of HIF-1α, which upregulates autophagy through activation of the downstream gene BNIP3 58 (Figure 1, ②; Figure 2, ②, ③, and ④).
HIF-1α promotes autophagy and attenuates apoptosis. 3,89HIF-1α also upregulates Bcl-2 expression. 90Beclin1 is downregulated by competing with Bcl-2 for interaction with Bnip3.HIF-1α was shown to enhance the migration and invasion of human endometriotic stromal cells through upregulation of autophagy and Bcl-2 expression 3,89 (Figure 1, ③), which may contribute to the pathogenesis of endometriosis by reducing the apoptosis of endometriotic cells.Thus, autophagy is thought of as a survival mechanism that can prevent cell death or apoptosis under hypoxia. 3Conversely, excessive autophagy can accelerate cell death.Indeed, ectopic endometrial tissues also show increased LC3 levels and decreased p62 levels, which lead to apoptosis 3 (Figure 1, ④).Therefore, autophagy may switch from a survival to a cell death program or vice versa based on the concentrations of Bcl-2, Beclin1, and Bnip3 in a hypoxic environment.

| Oxidative stress
The accumulation of high levels of hemoglobin, heme, and iron is caused by cyclic bleeding found in patients with endometriosis. 91me is catabolized to biliverdin, carbon monoxide, and iron by the heme oxygenase enzyme system. 92In fact, the concentrations of iron within endometriosis cysts have been reported to vary between 65.3 and 1046.3 mg/L, demonstrating that the degree of iron-induced oxidative stress also varies depending on endometriotic foci. 90Excess iron accumulated in ovarian endometriotic lesions is strongly associated with an oxidative stress-induced autophagic stimulus 93 (Figure 1, ⑤).Iron excess is believed to generate high levels of ROS and oxidative stress through the Fenton reaction, which significantly inhibits cell proliferation and causes cell death. 94Also, iron overload causes extensive apoptosis and induces cytoplasmic vacuolization as the morphological changes along with increased LC3-II levels (Figure 1, ④). 95 Therefore, the overactivation of autophagy and apoptosis under oxidative stress may negatively impact normal endometrial growth and the development of endometriosis.
On the other hand, activation of autophagy may also exert cytoprotective properties in endometriosis. 67Indeed, oxygen-derivedfree radicals formed by excess labile iron are important modulators of Nrf2, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), AMP-activated protein kinase (AMPK), Akt, and p53. 94For example, activation of the Nrf2 pathway upregulates the expression of cytoprotective genes and antioxidant genes, reduces the level of ROS, protects cells from ROS, and blocks apoptosis 94,96 (Figure 1, ⑥).Furthermore, autophagy was shown to promote the survival of eutopic endometrial stromal cells by reducing ROS generation possibly through mediating mitochondrial quality control 93 (Figure 1, ⑥ and ⑦).In addition, iron overload causes activation of protective autophagy and inhibition of apoptosis in eutopic endometrial stromal cells through upregulation of sirtuin 1 (SIRT1) expression 93 (Figure 1, ⑦ and ③).SIRT1 participates in the regulation of autophagy through deacetylation of specific autophagy-related proteins (e.g., Beclin1 and LC3). 97Altered expression of autophagy-related molecules (e.g., Bcl-2, Bax, Beclin1, Bnip3, Parkin, LC3, and p62) in endometriotic cells can switch the signaling from pro-apoptosis to antiapoptosis (Figure 1, ④ vs ③).Therefore, contradictory findings may exist in endometriosis, as iron-induced oxidative stress exhibits a dual role in cell-fate determination, i.e., cell death and cytoprotection, based on the concentration of iron in endometriotic cysts.

| Estrogen
Estrogen plays a crucial role in the pathogenesis of endometriosis.Genes involved in the initiation and regulation of autophagy (e.g., mTOR, CXCR4, and ESR1) are upregulated in endometriosis. 73trogen receptor signaling activates signaling pathways such as PI3K/ AKT/mTOR and C-X-C motif chemokine ligand 12 (CXCL12)-CXCR4 axis. 73Therefore, estrogen suppresses autophagy through mTOR signal activation and CXCL12/CXCR4 interaction and promotes endometrial stromal cell proliferation 73 (Figure 1, ①).Mechanistically, similar to mTOR, CXCL12 may inhibit autophagy by reducing Beclin1 expression, reducing LC3B-I to LC3B-II conversion, increasing p62 levels, and downregulating autophagosome. 98High estrogen levels and progesterone resistance, a hallmark of endometriosis, may be involved in the downregulation of autophagy. 5Therefore, estrogen could decrease the apoptosis through the inhibition of autophagy.

| AMPK
Endometriotic cells adapt to various environmental stressors and survive under nutrient deprivation and starvation.AMPK is a metabolic sensor that responds to low cellular energy stores, thereby allowing the cell to maintain energy homeostasis. 99Activation of AMPK inhibits mTOR via inactivation of the PI3K/Akt signaling pathway 99,100 (Figure 1, ①).AMPK induces autophagy by inhibiting mechanistic target of rapamycin complex 1 (MTORC1), activating the ULK1 complex, and phosphorylating Beclin1. 101,102AMPK-mediated autophagy is a cytoprotective mechanism that increases nutrient and energy demand. 99

| P53
p53 is thought to inhibit mTOR activity, leading to the induction of autophagic and apoptotic pathways (Figure 1, ① and ②). 11,103,104creased p53 expression in ovarian endometrioma suggests inhibition of apoptosis. 76

| Others
In addition to these pathways or stressors, many genes that control autophagy and apoptosis in endometriosis have been reported.For example, Mst1 is known as a major growth suppressor involved in cancer invasion, proliferation, and apoptosis. 51Mst1 activates mitochondrial fission and inhibits mitophagy through enhancing Drp1 activation and repressing p53-mediated Parkin transcription activity. 51wnregulation of Mst1 expression in endometriosis has been reported to promote endometriotic cell proliferation through activation of mitophagy. 51Yes-associated protein (YAP), a core effector component of the Hippo signaling pathway, is associated with organogenesis, malignancy, and endometriosis. 105The Hippo-YAP pathway promotes cell proliferation in endometriotic stromal cells through inhibition of autophagy and apoptosis. 86The cyclic GMP-AMP synthase (cGAS)stimulator of interferon genes (STING) pathway promotes immune effector responses associated with tumorigenesis. 106Activation of the cGAS-STING signaling pathway in endometriosis causes cell proliferation through induction of autophagy. 107The tumor suppressor DIRAS family GTPase 3 (DIRAS3) is a physiological autophagy inducer. 108regulation of DIRAS3 expression in endometriosis promotes proliferation of endometriotic epithelial cells through activating autophagy and inhibiting apoptosis. 109The tumor suppressor programmed cell death 4 (PDCD4) suppresses tumor progression. 110PDCD4 partially suppresses endometriosis cell proliferation and invasion through inhibition of autophagy. 110

| The relationship between autophagy and apoptosis in animal models of endometriosis
Animal models are essential to better understand the molecular mechanisms involved in autophagy and apoptosis in endometriosis. 15,50,75,77,107,111,112In this section, rather than providing a detailed overview of animal models of endometriosis, we explain the contradictory role of autophagy.Most experimental endometriosis was induced by transplantation of normal uterine tissue into the peritoneal cavity. 15,75The autophagic pathway was altered in the endometriosis-like lesions as compared with eutopic endometrium and normal endometrium. 75In a rat model of endometriosis, the expression of mTOR was increased, and the expression of Beclin1, Bnip3, Ambra1, LC3II, and Parkin was decreased, 15 suggesting that the autophagy level is decreased in ectopic endometrium compared to eutopic endometrium and normal endometrium.
Conversely, there were also reports that autophagy markers (e.g., Beclin1 and LC3B) were elevated in the ectopic endometrium and further increased in the eutopic endometrium from a mouse model of endometriosis compared to controls. 75This model suggests that activation of autophagy in endometriotic cells may favor apoptosis inhibition.Collectively, similar to in vitro and clinical data, the resulting data in animal models of endometriosis are also inconsistent.Interestingly, changes in Beclin1 and Bnip3 expression were also identified in animal models.

| MODUL ATING AUTOPHAGY A S A THER APEUTI C S TR ATEGY FOR ENDOME TRIOS IS
Only one paper was found that discussed pharmacological modulation of mitophagy and the therapeutic potential of targeting mitochondrial dynamics in endometriosis. 51Therefore, this section summarizes therapeutic potential of targeting autophagy in the treatment of endometriosis.Accumulating evidence indicated that pharmacological modulation of autophagy attenuated the progression of endometriosis in both in vitro and in vivo settings.However, as shown below, research has shown varied results on the autophagymediated apoptosis properties, including that autophagy-mediated apoptosis induction inhibited the development and progression of endometriotic lesions, 15,50,74,112 whereas autophagy inhibition significantly reduced the proliferation of endometriotic cells through promoting apoptosis. 3,67,75veral papers have reported that autophagy results in apoptosis induction in endometriotic cells. 15,50,74,112First, rapamycin, a macrocyclic antibiotic isolated from Streptomyces hygroscopicus, is an immunosuppressant, antifungal, and antitumor drug that blocks mTOR protein kinase. 113Induction of autophagy by rapamycin affects apoptosis in a variety of cell types. 33Choi et al reported that rapamycin induces autophagy and further promotes apoptosis in endometriotic cyst stromal cells. 74Mechanistically, the mTOR inhibition acts as a hub to activate autophagy mechanisms through increased expression of Beclin1, LC3II, Bnip3, Ambra1, and Parkin in a rat model of endometriosis. 15Furthermore, rapamycin-mediated activation of mitophagy induces apoptosis by activating proapoptotic Bcl-2 family proteins (e.g., Bax), inducing cytochrome c release, and then promoting caspase activation. 30,74Thus, upregulated autophagy triggered by rapamycin can promote apoptosis. 74,81Although rapamycin can provide a therapeutic treatment for endometriosis, its efficacy may be limited because of toxicity, side effects, and a ubiquitous expression of mTOR.
Second, the search for and development of novel mTOR modulators with fewer side effects to treat endometriosis remains a challenge.In recent years, plant-based natural products have attracted attention.Açai Berry is an Amazon's popular functional food produced by the Euterpe oleracea palm and has been reported as a molecule that modulates the autophagy pathway. 114Açai Berry has demonstrated its efficacy in animal models of endometriosis. 50ai Berry inhibits PI3K/Akt/extracellular-regulated MAP kinase (ERK)1/2/mTOR signals, promotes the activity of ULK1/Beclin1/ Ambra1 molecules, and enhances the processes of the autophagosome nucleation, expansion, and maturation. 50Furthermore, it promotes apoptosis through upregulating Bax expression and downregulating Bcl-2 expression. 50ird, increased estrogen receptor expression and decreased progesterone receptor isoform B expression are negative regulators of apoptosis and autophagy in endometriotic cells. 112For example, SCM-198 is the synthetic compound of leonurine, an alkaloid found in Herba leonuri, and has an antiestrogenic property. 112SCM-198 promotes endometriosis cell death via upregulation of apoptosis. 112Furthermore, dienogest treatment induces autophagy and promotes apoptosis through inhibition of Akt/ ERK1/2/mTOR signaling in endometriotic cells. 83Thus, autophagy induction via mTOR inhibition is emerging as a promising therapeutic strategy for endometriosis.
Conversely, autophagy inhibition may also promise to be an effective strategy for treatment of endometriosis.
Hydroxychloroquine (HCQ), an autophagy inhibitor, is an alkalinizing lysosomotropic agent that have been used for the treatment of malaria and various autoimmune diseases such as systemic lupus erythematosus and rheumatic and dermatologic diseases. 75,115filomycin A1, a chemical inhibitor of lysosomal proton pump vacuolar-type ATPase (V-ATPase), prevents the formation of autophagosomes, blocks autophagosome-lysosome fusion, inhibits autolysosome acidification, and disrupts autophagic flux, leading to autophagy inhibition. 116Treatment with hydroxychloroquine 75 significantly decreased endometriotic cell growth through promoting apoptosis.Additionally, bafilomycin A1 inhibited autophagy in endometriotic stromal cells. 117In addition, HIF-1α promotes the migration and invasion of endometrial stromal cells through upregulation of the expression of autophagy-related molecules. 3eonol, 2′-hydroxy-4′-methoxyacetophenone, is a bioactive phenol present in the root bark of the Moutan Cortex 118 and downregulates the HIF-1α-mediated pathway proteins. 67HIF-1α-mediated autophagy inhibition by Paeonol suppresses both migration and invasion in endometriotic cells. 67llectively, HIF-1α and mTOR act as a positive and negative regulator of autophagy, respectively, in endometriosis.The inactivation of the PI3K/Akt/mTOR pathway can induce autophagy and subsequently promotes apoptosis.Conversely, the inactivation of the HIF-1α pathway can promote apoptosis through inhibiting autophagy.These findings support the view that fine-tuning the induction and inhibition of autophagy can be used as an effective intervention strategy for endometriosis treatment.

| DISCUSS ION
We highlighted studies that have evaluated alterations in autophagy/mitophagy and apoptosis in endometriosis and provided an overview regarding its pathogenesis and personalized treat- to adapt to ever-changing environments, such as the regulation of signal pathway and oxygen, iron, and nutrient concentrations (Figure 1, ① and ⑤).Upregulation of mTOR expression can suppress autophagy, 15,33,50,61,73,74,100,112 whereas hypoxia and iron-mediated oxidative stress often promote autophagy. 3,25,62For example, inappropriate activation of the PI3K/Akt/ERK1/2/mTOR pathway leads to inhibition of autophagy and subsequent suppression of apoptosis (Figure 1, ①).Inhibition of autophagy suppresses apoptosis and promotes endometriotic cell proliferation.Indeed, some drugs, such as dienogest, 83 rapamycin, 15,33,74 Açai Berry, 50 and HCQ, 75 have been reported to exhibit apoptosis promoting and growth suppressive effects on endometriosis by promoting autophagy.On the other hand, previous studies have also shown that ovarian endometriotic cells upregulate autophagy to survive and promote growth. 76,119In fact, activation of the HIF-1α pathway and excess ROS induces autophagy and avoids cell death through antiapoptotic effects (Figure 1, ②, ⑤, ⑦, and ③).7][28] Therefore, endometriotic cells must rely on autophagy to survive external challenges such as hypoxia and oxidative stress.
However, HIF-1α-or ROS-induced overactivation of autophagy has also been reported to induce cell death via promoting apoptosis (Figure 1, ④).Therefore, autophagy may play a dual role in suppressing and promoting the development of endometriosis.
We discuss why these seemingly contradictory results occur.
First, regarding the mechanism by which autophagy controls apoptosis, alternative or compensatory mechanisms need to be considered.For example, oxidative stress and nutrient starvation induce the expression of SIRT1 120 and AMPK 121 as compensatory mechanisms as a means of coping with harsh environments.These molecules have key roles in the regulation of cellular metabolism and homeostasis.Key signaling pathways and downstream target molecules involved in autophagy are significantly altered in each endometriosis to deal with ever-changing environments. 5Second, autophagy and apoptosis are tightly regulated processes that share a common signal. 6,15,29,122[61][62][63][64] Beclin1 and Bnip3 can bind Bcl-2 in a mutually exclusive manner and are involved in determining autophagy and apoptosis (see Section 4).Therefore, apoptosis may be promoted or inhibited depending on the concentrations of Bcl-2, Bax, Beclin1, and Bnip3 proteins in endometriotic lesions (Figure 2).

FU N D I N G I N FO R M ATI O N
No funding was received.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare no competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
No new data were created.

E TH I C S S TATEM ENT
Ethics Approval The submitted paper is a review article and has not been approved by the Institutional Review Board and the Research and Ethical Committee of Nara Medical University Graduate School of Medicine, Kashihara, Japan.
of current understanding of signaling pathways that control autophagy, mitophagy, and apoptosis in endometriosis.Route 1, The interplay between autophagy and apoptosis through shared signaling pathways (e.g., PI3K/AKT/mTOR pathway, estrogen signals, AMPK, or p53); Route 2, Induction of HIF-1α stabilization by hypoxia; Route 3, Suppression of apoptosis by upregulation of Bcl-2 expression; Route 4, Apoptosis induced by excessive autophagy; Route 5, Autophagy regulated by iron overload; Route 6, Suppression of apoptosis by upregulation of antioxidant gene expression; and Route 7, Regulation of autophagy by SIRT1-dependent deacetylation.

F I G U R E 2
An overview of the molecular mechanism of mitophagy in endometriosis.Route 1, ubiquitin-dependent mitophagy pathway; Route 2, Inhibition of PINK1 degradation by Bnip3; Route 3, Parkin translocation to mitochondria by Bnip3; Route 4, The ubiquitin-independent pathway orchestrated by a receptor protein, BNIP3; Route 5, The interrelationship between molecules, Bcl-2, Bnip3, and Beclin1: Bnip3 competes with Beclin1 for the binding site of Bcl-2 protein and releases Beclin1; Route 6, Beclin1 is involved in autophagosome elongation, maturation, and autophagolysosome formation in the ubiquitin-dependent pathway; Route 6′, Beclin1 is involved in autophagosome elongation, maturation, and autophagolysosome formation in the ubiquitin-independent pathway; Route 7, The interrelationship between molecules, Bcl-2, Bnip3, and Beclin1: Beclin1 competes with Bnip3 for the binding site of Bcl-2 protein and releases Bnip3; Route 8, Excessive Bcl-2 downregulates mitophagy through binding to Bnip3; and Route 9, Apoptosis induction by Bnip3 via inhibition of Bcl-2.The pathways indicated by the black arrow has been proven to contribute to the process of mitophagy in endometriosis.The gray arrow indicates that despite the demonstrated effects of mitophagy on some other cells, there is no direct evidence for this reasoning in endometriotic cells.
ment strategies.Autophagy and mitophagy have been reported to control apoptosis through various pathways and contribute to promoting the development and progression of endometriosis.5 Since endometriosis is characterized by iron-mediated oxidative stress, mitophagy has been studied in the context of regulatory networks that coordinate mitochondrial quality control and antioxidant capacity.However, research on mitophagy in endometriosis is limited, and only a few studies have addressed the ubiquitin-independent mitophagy pathway.Although there is evidence suggesting that ubiquitin-independent pathway may be a critical factor in mitophagy, their role in the development, progression, and pathogenesis of endometriosis remains unclear.Therefore, this review mainly focuses on autophagy.Previous studies on the contradictory outcomes of the autophagy-mediated apoptosis have facilitated an elucidation of the underlying mechanisms regulating autophagy and apoptosis.Endometriotic cells can switch their autophagic responses from inhibitory to promoting mechanism or vice versa